Electrolyte for lithium-sulfur battery, and lithium-sulfur battery comprising same

Abstract

The present invention provides: an electrolyte for a lithium-sulfur battery, the electrolyte comprising a predetermined lithium salt, thereby exhibiting excellent capacity retention and lifespan characteristics; and a lithium-sulfur battery comprising same. The electrolyte for a lithium-sulfur battery, according to one embodiment of the present invention, comprises a lithium salt, a nonaqueous solvent and an additive, wherein the lithium salt comprises lithium bis(fluorosulfonyl)imide (LiFSI) and a triflate-based compound, and the total molar concentration of LiFSI and the triflate-based compound in the electrolyte is 0.5-1.0 M.

Description

Electrolyte for lithium-sulfur batteries and lithium-sulfur batteries containing the same

[0001] The present invention relates to an electrolyte for a lithium-sulfur battery and a lithium-sulfur battery including the same.

[0002] This application claims priority to Korean Application No. 10-2023-0172599, filed December 1, 2023, the entire disclosure of which is incorporated herein by reference.

[0003] A lithium-sulfur battery is a battery system that uses a sulfur-based material containing a sulfur-sulfur bond as a positive electrode active material, and lithium metal, a carbon-based material in which lithium ions can be inserted / deinserted, or silicon or tin that forms an alloy with lithium as a negative electrode active material.

[0004] Sulfur, the main material of the cathode active material in lithium-sulfur batteries, has the advantages of being a low atomic weight, abundant in resources, easy to supply, inexpensive, non-toxic, and environmentally friendly material.

[0005] In addition, lithium-sulfur batteries have a conversion reaction between lithium ions and sulfur (S8+16Li) at the cathode. + +16e - → The theoretical specific capacity from 8Li2S reaches 1,675 mAh / g, and when lithium metal is used as the negative electrode, it shows a theoretical energy density of 2,600 Wh / kg. This is a very high figure compared to the theoretical energy density of other battery systems currently being studied (Ni-MH battery: 450 Wh / kg, Li-FeS battery: 480 Wh / kg, Li-MnO2 battery: 1,000 Wh / kg, Na-S battery: 800 Wh / kg) and lithium ion batteries (250 Wh / kg), and therefore, it is attracting attention as a high-capacity, eco-friendly, and low-cost lithium secondary battery among the secondary batteries being developed so far.

[0006] When discharging a lithium-sulfur battery, a reduction reaction occurs at the positive electrode where sulfur accepts electrons, and an oxidation reaction occurs at the negative electrode where lithium ionizes. Specifically, when discharging, lithium, the negative active material, is oxidized as it donates electrons and ionizes into lithium cations, and the sulfur-based material, the positive active material, is reduced as it accepts electrons. Here, through the reduction reaction of the sulfur-based material, the S-S bond accepts two electrons and is converted into a sulfur anion. The lithium cation generated by the lithium oxidation reaction is transferred to the positive electrode through the electrolyte, and this combines with the sulfur anion generated by the reduction reaction of the sulfur-based compound to form a salt. Specifically, before discharging, sulfur has a cyclic S8 structure, which is converted into lithium polysulfide (Li2Sx) through the reduction reaction, and is completely reduced to produce lithium sulfide (Li2S).

[0007] At this time, lithium polysulfide (Li2S) generated at the positive electrode x , x=2~8) are easily dissolved in the electrolyte and cause side reactions within the battery, which accelerates the deterioration of the battery and reduces its lifespan. In addition, problems such as a shuttle reaction occurring during the charging process and a significant decrease in the charge / discharge efficiency are occurring.

[0008] In addition, there are attempts to suppress the above problems by using LiFSI as a lithium salt or using a nitrate compound as an additive, but these are consumable additives and continuously decompose, so problems such as reduced lifespan still exist.

[0009] Accordingly, the inventors of the present invention have conducted multifaceted research to solve the above problems, and as a result, the purpose is to provide an electrolyte for a lithium-sulfur battery that can prevent deterioration of the negative electrode due to lithium polysulfide by introducing LiFSI and a triflate compound as a lithium salt.

[0010] In addition, the purpose is to provide a lithium-sulfur battery with increased capacity retention and improved lifespan characteristics.

[0011] To achieve the above purpose,

[0012] According to one aspect of the present invention, an electrolyte for a lithium sulfur battery of the following embodiments is provided.

[0013] According to the first implementation example,

[0014] An electrolyte for a lithium-sulfur battery comprising a lithium salt, a non-aqueous solvent, and an additive.

[0015] The lithium salts include LiFSI (Lithium bis(fluorosulfonyl)imide) and triflate compounds.

[0016] The present invention relates to an electrolyte for a lithium-sulfur battery, wherein the total molar concentration of the LiFSI (Lithium bis(fluorosulfonyl)imide) and triflate compound among the electrolytes is 0.5 to 1.0 M.

[0017] According to the second embodiment, in the first embodiment,

[0018] The present invention relates to an electrolyte for a lithium-sulfur battery, characterized in that the molar concentration of the triflate compound in the electrolyte is 0.05 to 0.30 M.

[0019] According to the third embodiment, in the first embodiment or the second embodiment,

[0020] The present invention relates to an electrolyte for a lithium-sulfur battery, characterized in that the total molar concentration of the LiFSI (Lithium bis(fluorosulfonyl)imide) and the triflate compound among the electrolytes is 0.6 to 0.95 M.

[0021] According to the fourth embodiment, in any one of the first to third embodiments,

[0022] The present invention relates to an electrolyte for a lithium-sulfur battery, characterized in that the concentration of the triflate compound in the electrolyte is 0.05 to 0.20 M.

[0023] According to the fifth embodiment, in any one of the first to fourth embodiments,

[0024] The above triflate compound relates to an electrolyte for a lithium sulfur battery, characterized in that it includes LiTf (lithium triflate).

[0025] According to the sixth embodiment, in any one of the first to fifth embodiments,

[0026] The above additive relates to an electrolyte for a lithium sulfur battery, characterized in that it contains a nitric acid compound.

[0027] According to the seventh embodiment, in the sixth embodiment,

[0028] The above-mentioned nitric acid compound relates to an electrolyte for a lithium sulfur battery, characterized in that it is lithium nitrate (LiNO3).

[0029] According to the eighth embodiment, in any one of the first to seventh embodiments,

[0030] The above non-aqueous solvent relates to an electrolyte for a lithium sulfur battery, characterized in that it includes an ether solvent.

[0031] According to the ninth embodiment, in the eighth embodiment,

[0032] The present invention relates to an electrolyte for a lithium-sulfur battery, characterized in that the ether solvent is included in a content of 80% by volume or more based on the total volume of the non-aqueous solvent.

[0033] According to the 10th embodiment, in the 8th embodiment,

[0034] The above ether solvent relates to an electrolyte for a lithium sulfur battery, characterized in that it comprises a linear ether, a cyclic ether, or a mixture thereof.

[0035] According to the 11th implementation example,

[0036] A lithium-sulfur battery comprising a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte,

[0037] The present invention relates to a lithium-sulfur battery, characterized in that the electrolyte is an electrolyte for a lithium-sulfur battery according to any one of the first to tenth embodiments.

[0038] According to the 12th embodiment, in the 11th embodiment,

[0039] The above positive electrode relates to a lithium sulfur battery characterized in that it includes a sulfur-containing compound as a positive electrode active material.

[0040] According to the 13th embodiment, in the 12th embodiment,

[0041] The above sulfur-containing compounds include inorganic sulfur (S8), lithium polysulfide (Li2S) n , 1≤n≤8), sulfur-carbon complex or a mixture of two or more thereof.

[0042] According to the 14th embodiment, in any one of the 11th to 13th embodiments,

[0043] The above negative electrode relates to a lithium-sulfur battery characterized in that it includes lithium metal, lithium alloy or a mixture thereof as a negative electrode active material.

[0044] The present invention not only protects the negative electrode surface with the lithium salt LiFSI, but also enhances the surface protection of the negative electrode by mixing in a stable triflate compound that is less decomposed than LiFSI. This prevents degradation of the negative electrode due to polysulfides and suppresses the accumulation of lithium sulfides on the negative electrode surface, thereby stabilizing the negative electrode surface.

[0045] In addition, the electrolyte for a lithium-sulfur battery of the present invention and the lithium-sulfur battery including the same can maintain the salt concentration of the electrolyte at a constant level, thereby exhibiting excellent capacity retention and exhibiting the effect of improved life characteristics.

[0046] The following drawings attached to this specification illustrate preferred embodiments of the present invention, and serve to further understand the technical idea of ​​the present invention together with the contents of the invention described above, and therefore the present invention should not be interpreted as being limited to matters described in such drawings.

[0047] Figures 1 to 5 are graphs showing the discharge capacity measured according to the number of cycles of lithium-sulfur batteries according to Examples 1 to 5 and Comparative Examples 1 to 8.

[0048] Hereinafter, the present invention will be described in more detail.

[0049] Terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best way.

[0050] The terminology used in this invention is used solely to describe specific embodiments and is not intended to limit the invention. Singular expressions include plural expressions unless the context clearly dictates otherwise.

[0051] Throughout this specification, whenever a part is said to “include” or “have” a component, this does not mean that other components are excluded, but rather that other components may be included, unless otherwise specifically stated.

[0052] The terms "about," "substantially," etc., used throughout this specification are used in the sense of, or close to, the numerical values ​​when manufacturing and material tolerances inherent to the meanings stated, and are used to prevent unscrupulous infringers from unfairly exploiting disclosures that state exact or absolute values ​​to aid understanding of this specification.

[0053] Throughout this specification, the description of “A and / or B” means “A or B or both.”

[0054] The term "composite" used in this specification refers to a material in which two or more materials are combined to form physically and chemically different phases and exhibit more effective functions.

[0055] The term “polysulfide” used in this specification means “polysulfide ion (S x 2- , x= 8, 6, 4, 2) and 「lithium polysulfide (Li2S x or Li2S x - = 8, 6, 4, 2) is a concept that includes all of them.

[0056]

[0057] Lithium secondary batteries, particularly lithium-sulfur batteries, suffer from the following problems: During charging and discharging, the lithium anode and electrolyte continuously react, resulting in the deterioration of the anode due to lithium sulfide and / or polysulfide. Lithium sulfide accumulates on the surface of the anode, which is detrimental to its lifespan. Furthermore, the continuous reaction between the anode and electrolyte accelerates the decomposition of lithium salts, resulting in poor stability. Therefore, one aspect of the present invention seeks to address the aforementioned problems.

[0058]

[0059] The electrolyte for a lithium sulfur battery of the present invention

[0060] Contains lithium salt, non-aqueous solvent and additives,

[0061] The above lithium salt includes LiFSI (Lithium bis(fluorosulfonyl)imide) and triflate compounds,

[0062] The total molar concentration of LiFSI (Lithium bis(fluorosulfonyl)imide) and triflate compounds in the above electrolyte is 0.5 to 1.0 M.

[0063]

[0064] In one embodiment of the present invention, the electrolyte may be an electrolyte solution, but is not limited thereto.

[0065]

[0066] lithium salt

[0067] In the present invention, the lithium salt includes LiFSI (Lithium bis(fluorosulfonyl)imide) and a triflate compound.

[0068] The above LiFSI (Lithium bis(fluorosulfonyl)imide) and triflate compounds can effectively perform surface protection of the negative electrode, thereby stabilizing the surface of the negative electrode and preventing degradation.

[0069] Previously, LiFSI was used as a lithium salt to protect the surface of the negative electrode, but there were problems such as continuous reaction between the negative electrode and the electrolyte, causing the decomposition of the lithium salt, and the decomposition of the lithium salt accelerating as the temperature increased. In addition, there were attempts to use a mixture of LiFSI and LiPF6 as lithium salts, but LiPF6 had the problem of not dissolving well in ether solvents.

[0070] Accordingly, in the present invention, a stable triflate compound that is less decomposed than LiFSI is mixed to replenish continuously consumed LiFSI, thereby protecting the surface of the negative electrode, assisting the transport process of lithium cations, and at the same time improving the life characteristics by stabilizing the salt concentration in the electrolyte.

[0071] In the present invention, the triflate compound may refer to a lithium triflate compound, and may be a triflate (R-OSO2CF3, R is an alkyl group), or a triflate ion (-OSO2CF3 - ) is not particularly limited. For example, in the present invention, the triflate compound may include a compound composed of a lithium cation and a triflate anion.

[0072] In another embodiment of the present invention, the lithium salt may further include other types of lithium salts, in addition to the LiFSI and triflate compounds, as long as they are used in the electrolyte of a lithium-sulfur battery, and are not particularly limited thereto, but the types thereof are not limited in the present specification.

[0073]

[0074] In the present invention, the total molar concentration of LiFSI (Lithium bis(fluorosulfonyl)imide) and the triflate compound, i.e., the sum total molar concentration of LiFSI (Lithium bis(fluorosulfonyl)imide) and the triflate compound, may be 0.5 to 1.0 M or 0.6 to 0.95 M in the electrolyte. When the above-described range is satisfied, it is advantageous to secure ionic conductivity suitable for battery operation, or the electrolyte may exhibit an appropriate viscosity, thereby improving the mobility of lithium ions and suppressing the decomposition reaction of the lithium salt itself, thereby exhibiting an advantageous effect in terms of capacity retention.

[0075] In one embodiment of the present invention, the molar concentration of the triflate compound may be 0.05 to 0.30 M, or 0.05 to 0.20 M, in the electrolyte. When the above-described range is satisfied, the movement of lithium ions is facilitated, thereby securing ionic conductivity suitable for battery operation, and the electrolyte exhibits an appropriate viscosity, thereby suppressing an increase in internal resistance. That is, when the molar concentration of the triflate compound is within an appropriate range, the mobility of lithium ions can be improved and the decomposition reaction of the lithium salt itself can be suppressed, thereby exhibiting a favorable effect in terms of capacity retention.

[0076]

[0077] additives

[0078] The additive of the present invention is not particularly limited as long as it does not participate in the electrochemical reaction of the battery and can play a role in improving the efficiency of the negative electrode and the performance of the battery.

[0079] In one embodiment of the present invention, the additive may include a nitric acid compound.

[0080] The above-mentioned nitric acid compound can improve the electrical conductivity of a lithium-sulfur battery by dissolving in the electrolyte of the lithium secondary battery in addition to the lithium salt and providing ions, and can also prevent irreversible consumption of polysulfides by suppressing the reduction reaction of polysulfides generated during the charge and discharge process of the lithium-sulfur battery, thereby improving the performance of the lithium-sulfur battery.

[0081] The above nitric acid compound may be used without particular limitations as long as it forms a stable film on the negative electrode of a lithium-sulfur battery and exhibits the effect of improving charge / discharge efficiency.

[0082] For example, the nitric acid compound may be an inorganic nitric acid or nitrous acid compound such as lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), barium nitrate (Ba(NO3)2), ammonium nitrate (NH4NO3), lithium nitrite (LiNO2), potassium nitrite (KNO2), cesium nitrite (CsNO2), ammonium nitrite (NH4NO2); an organic nitric acid or nitrous acid compound such as methyl nitrate, dialkyl imidazolium nitrate, guanidine nitrate, imidazolium nitrate, pyridinium nitrate, ethyl nitrite, propyl nitrite, butyl nitrite, pentyl nitrite, or octyl nitrite; It may be selected from the group consisting of organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, dinitrotoluene, and combinations thereof, and preferably may include lithium nitrate.

[0083]

[0084] In one embodiment of the present invention, the content of the additive may be included in an amount of 1 wt% to 10 wt%, 2 wt% to 9 wt%, 3 wt% to 8 wt%, or 3 wt% to 5 wt% based on 100 wt% of the total electrolyte for a lithium-sulfur battery, but is not limited thereto. When the additive is included in the above-described amount, it is advantageous in terms of improving the electrical conductivity of the electrolyte and suppressing the reduction of polysulfide when used in a lithium-sulfur battery.

[0085]

[0086] non-aqueous solvent

[0087] The above non-aqueous solvent may be used without particular limitation as long as it is used in the electrolyte of a lithium-sulfur battery, and is for dissolving the lithium salt and / or the additive.

[0088] For example, the non-aqueous solvent may include an ether solvent.

[0089] The above ether solvent may include, for example, linear ethers, cyclic ethers, or mixtures thereof, depending on the structure of the compound. In one embodiment of the present invention, the linear ether is selected from the group consisting of, for example, dimethyl ether, diethyl ether, dipropyl ether, dibutyl ether, diisobutyl ether, ethyl methyl ether, ethylpropyl ether, ethyl tertbutyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, dimethoxypropane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, dipropylene glycol dimethylene ether, butylene glycol ether, diethylene glycol ethyl methyl ether, diethylene glycol isopropyl methyl ether, diethylene glycol butyl methyl ether, diethylene glycol tertbutyl ethyl ether, and ethylene glycol ethyl methyl ether. It may include one or more kinds. Preferably, it may include one or more kinds selected from the group consisting of dimethyl ether, dimethoxyethane, diethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether, and more preferably, it may include dimethoxyethane.

[0090] In one embodiment of the present invention, the cyclic ether is, for example, 2-methylfuran, 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-dioxane, It may include at least one selected from the group consisting of 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, and isosorbide dimethyl ether. Preferably, it may include at least one selected from the group consisting of 2-methylfuran, 1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, and 2,5-dimethyltetrahydrofuran, and more preferably, it may include 2-methylfuran.

[0091] In one embodiment of the present invention, the non-aqueous solvent may include dimethoxyethane and 2-methylfuran.

[0092]

[0093] In one embodiment of the present invention, the non-aqueous solvent may comprise an ether solvent in an amount greater than or equal to a majority of the total volume of the non-aqueous solvent. Specifically, the ether solvent may comprise 50% by volume or more, specifically 60% by volume or more, and more specifically 70% to 100% by volume, or 80% to 100% by volume, based on the total volume of the non-aqueous solvent.

[0094] In another embodiment of the present invention, the non-aqueous solvent may include a linear ether and a cyclic ether. For example, the non-aqueous solvent may include dimethoxyethane (DME) as a linear ether and 2-methyl furan (2-MeF) as a cyclic ether.

[0095] According to one embodiment of the present invention, when the non-aqueous solvent includes a linear ether and a cyclic ether, the linear ether and the cyclic ether may be included in a volume ratio of, for example, 1:5 to 5:1 or 1:4 to 4:1, but the present invention is not limited thereto.

[0096] As described above, by using the composition of an electrolyte for a lithium-sulfur battery according to one aspect of the present invention, it is possible to suppress the dissolution of lithium polysulfide, improve the passivation phenomenon caused by the accumulation of lithium sulfide on the negative electrode, and thereby improve the operating stability of a lithium-sulfur battery. However, the present invention is not limited thereto.

[0097]

[0098] In addition, in addition to the above ether solvent, an ester solvent, a carbonate solvent, or two or more solvents thereof may be additionally included.

[0099] In one embodiment of the present invention, the ester solvent may be, for example, one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, or a mixture of two or more thereof, but is not limited thereto.

[0100] In one embodiment of the present invention, the carbonate solvent may be, for example, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate and halides thereof, or a mixture of two or more thereof. Examples of the halides thereof include, but are not limited to, fluoroethylene carbonate.

[0101] In another embodiment of the present invention, since the carbonate solvent does not dissolve the nitric acid compound or exhibits low solubility, the non-aqueous solvent may substantially not contain the carbonate solvent.

[0102]

[0103] lithium-sulfur battery

[0104] In addition, according to another aspect of the present invention, a lithium-sulfur battery comprising the electrolyte for a lithium-sulfur battery described above is provided.

[0105] The above lithium-sulfur battery relates to a lithium secondary battery including a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, wherein the electrolyte is the same as the electrolyte for the lithium-sulfur battery of the present invention described above.

[0106]

[0107] anode

[0108] The above positive electrode may include a positive electrode current collector and a positive electrode active material layer applied to one or both sides of the positive electrode current collector.

[0109] The above-mentioned positive electrode current collector supports the positive electrode active material and is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the positive electrode active material layer includes the positive electrode active material and may further include a conductive agent, a binder, an additive, and the like.

[0110] The above-described positive electrode active material comprises a porous carbon material and a sulfur-carbon composite containing sulfur on at least a portion of the inner and outer surfaces of the porous carbon material. Since the sulfur contained in the positive electrode active material does not have electrical conductivity on its own, it is used in a composite form with a conductive material such as a carbon material. Accordingly, the sulfur is contained in the form of a sulfur-carbon composite. The sulfur may include at least one selected from the group consisting of elemental sulfur (S8) and sulfur compounds.

[0111] The above positive electrode active material may include inorganic sulfur, lithium polysulfide (Li2Sn, 1≤n≤8), a sulfur-carbon complex, or a mixture of two or more thereof.

[0112] The above sulfur-carbon complex not only provides a framework in which the aforementioned sulfur can be uniformly and stably fixed, but also includes a porous carbon material to complement the low electrical conductivity of sulfur so that an electrochemical reaction can proceed smoothly.

[0113] The above porous carbon material can generally be manufactured by carbonizing precursors of various carbon materials. The porous carbon material includes pores that are not uniform in size, and the average diameter of the pores ranges from 1 to 200 nm, and the porosity can range from 10 to 90% of the total volume of the porous carbon material. If the average diameter of the pores is less than the above range, the pore size is only at the molecular level, making sulfur impregnation impossible. Conversely, if it exceeds the above range, the mechanical strength of the porous carbon material is weakened, making it undesirable for application to the electrode manufacturing process.

[0114] In one embodiment of the present invention, the 'average pore diameter' can be measured according to a method known in the art for measuring the pore diameter of a porous material, and the measurement method is not particularly limited. For example, the pore diameter can be measured according to a scanning electron microscope (SEM), a field emission electron microscope (laser diffraction method), or a laser diffraction method. The measurement using the laser diffraction method can be, for example, using a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000).

[0115] In one embodiment of the present invention, the 'porosity' refers to the ratio of the volume occupied by pores to the total volume in a certain structure, and uses % as its unit, and can be used interchangeably with terms such as porosity, porosity, etc. In the present invention, the measurement of the porosity is not particularly limited, and according to one embodiment of the present invention, for example, it can be measured according to the BET (Brunauer-Emmett-Teller) measurement method using nitrogen gas or the mercury penetration method (Hg porosimeter) and ASTM D2873.

[0116] The shape of the above porous carbon material may be spherical, rod-shaped, needle-shaped, plate-shaped, tubular or bulk-shaped, and can be used without limitation as long as it is a shape commonly used in lithium-sulfur batteries.

[0117] The porous carbon material may be any material commonly used in the art that has a porous structure or a high specific surface area. For example, the porous carbon material may be at least one selected from the group consisting of graphite; graphene; carbon black such as Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs); and graphite and activated carbon such as natural graphite, artificial graphite, and expanded graphite, but is not limited thereto. Preferably, the porous carbon material may be a carbon nanotube.

[0118] In the sulfur-carbon composite according to the present invention, the sulfur is located on at least one of the inner and outer surfaces of the porous carbon material, and for example, may be present in an area less than 100%, preferably 1 to 95%, and more preferably 40 to 96% of the entire inner and outer surfaces of the porous carbon material. When the sulfur is present on the inner and outer surfaces of the porous carbon material within the above range, it can exhibit the maximum effect in terms of electron transfer area and wettability with the electrolyte. Specifically, since the sulfur is thinly and evenly impregnated on the inner and outer surfaces of the porous carbon material within the above-described range, the electron transfer contact area can be increased during the charge and discharge process. If the sulfur is located in an area of ​​100% of the entire inner and outer surfaces of the porous carbon material, the porous carbon material is completely covered with sulfur, and the wettability and contact with the electrolyte are reduced, so that electron transfer is not possible and the porous carbon material cannot participate in the electrochemical reaction.

[0119] The above sulfur-carbon complex may contain sulfur in an amount of, for example, 65 wt% or more, specifically 65 to 90 wt%, 70 to 85 wt%, or 72 to 80 wt%, based on 100 wt% of the sulfur-carbon complex. When the sulfur content is within the above-described range, it may exhibit advantageous effects in terms of improving battery performance and securing battery capacity, but the present invention is not limited thereto.

[0120] The method for manufacturing the sulfur-carbon composite of the present invention is not particularly limited, and any method commonly used in the art may be used. For example, a method of simply mixing the sulfur and porous carbon material and then heat-treating to form a composite may be used.

[0121] In addition to the composition described above, the positive electrode active material may further include one or more selected from among a transition metal element, a group ⅢA element, a group ⅣA element, a sulfur compound of these elements, and an alloy of these elements and sulfur.

[0122] The above transition metal elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au or Hg, etc., the above group ⅢA elements include Al, Ga, In, Ti, etc., and the above group ⅣA elements may include Ge, Sn, Pb, etc.

[0123] In the positive electrode of the lithium-sulfur battery of the present invention, the positive electrode active material may be included in an amount of, for example, 80 wt% or more, specifically 80 wt% to 100 wt%, more specifically 85 wt% to 98 wt%, or 80 wt% to 95 wt%, based on the total weight of the positive electrode active material layer. The content of the positive electrode active material may have a lower limit of 70 wt% or more or 85 wt% or more, and an upper limit of 99 wt% or less or 90 wt%, based on 100 wt% of the total positive electrode active material layer. The content of the positive electrode active material may be set by a combination of the lower limit and the upper limit. When the content of the positive electrode active material is less than the above range, the relative content of auxiliary materials such as conductive materials and binders increases and the content of the positive electrode active material decreases, making it difficult to realize a battery with high capacity and high energy density. On the contrary, when it exceeds the above range, the content of the conductive material or binder described later is relatively insufficient, which causes a problem in that the physical properties of the electrode deteriorate.

[0124] The above conductive material is a material that electrically connects the electrolyte and the positive electrode active material and serves as a path for electrons to move from the current collector to the positive electrode active material. Any conductive material that is physically distinct from the carbon contained in the sulfur-carbon complex and is a component of the electrode can be used without limitation.

[0125] For example, the conductive material may include carbon black such as Super-P, Denka Black, Acetylene Black, Ketjen Black, Channel Black, Furnace Black, Lamp Black, Summer Black, and carbon black; carbon derivatives such as carbon nanotubes or fullerene; conductive fibers such as carbon fibers or metal fibers; metal powders such as fluorocarbon, aluminum, and nickel powder; or conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole, which may be used alone or in combination.

[0126] The content of the conductive agent may be 1 to 10 wt% based on the total weight of the positive electrode active material. If the content of the conductive agent is below the above range, electron transfer between the positive electrode active material and the current collector is not easy, resulting in decreased voltage and capacity. Conversely, if the content exceeds the above range, the proportion of the positive electrode active material may relatively decrease, thereby reducing the total energy (charge) of the battery. Therefore, it is preferable to determine an appropriate content within the above-mentioned range.

[0127] The above binder maintains the positive electrode active material on the positive electrode current collector and organically connects the positive electrode active materials to increase the bonding strength between them, and any binder known in the industry can be used.

[0128] For example, the binder may be a fluororesin binder including polyvinylidene fluoride (PVdF), a polyvinylidene fluoride polymer including at least one vinylidene fluoride as a repeating unit, polytetrafluoroethylene (PTFE), or a mixture of two or more thereof; a rubber binder including styrene butadiene rubber (SBR), acrylonitrile-butidiene rubber, or styrene-isoprene rubber; an acrylic binder; a cellulose binder including carboxyl methyl cellulose (CMC), starch, hydroxy propyl cellulose, or regenerated cellulose; a polyalcohol binder; a polyolefin binder including polyethylene or polypropylene; a polyimide binder; a polyester binder; and a silane-based binder; a mixture or copolymer of one or more selected from the group consisting of;

[0129] The content of the above binder may be 1 to 10 wt% based on the total weight of the positive electrode active material layer. If the content of the binder is less than the above range, the physical properties of the positive electrode may deteriorate, causing the positive electrode active material and conductive material to fall off. If the content of the binder is more than the above range, the ratio of the positive electrode active material and conductive material in the positive electrode may relatively decrease, resulting in a decrease in battery capacity. Therefore, it is preferable to determine an appropriate content within the above-described range.

[0130] In the present invention, the method for manufacturing the positive electrode of the lithium sulfur battery is not particularly limited, and various methods known to those skilled in the art or modified methods thereof can be used.

[0131] For example, the positive electrode of the lithium sulfur battery may be manufactured by forming the positive electrode active material layer by preparing a positive electrode slurry composition including the composition described above and then applying the same to at least one surface of the positive electrode current collector.

[0132] The above positive electrode slurry composition includes the positive electrode active material described above, and may further include a binder, a conductive material, and a solvent.

[0133] The above solvent is one that can uniformly disperse the positive electrode active material. Water is most preferably an aqueous solvent, and the water may be distilled or deionized water. However, this is not necessarily limited to this, and if necessary, a lower alcohol that is easily mixed with water may be used. Examples of the lower alcohol include methanol, ethanol, propanol, isopropanol, and butanol, and preferably, these can be mixed with water and used.

[0134] The content of the above solvent may be contained at a level that has a concentration that can facilitate coating, and the specific content varies depending on the application method and device.

[0135] The above-mentioned positive electrode slurry composition may additionally contain, as necessary, substances commonly used in the relevant technical field for purposes such as improving its function. Examples thereof include viscosity modifiers, fluidizing agents, and fillers.

[0136] The method for applying the positive electrode slurry composition is not particularly limited in the present invention, and examples thereof include methods such as doctor blade, die casting, comma coating, and screen printing. In addition, the positive electrode slurry may be applied onto the positive electrode current collector by molding it on a separate substrate and then pressing or lamination.

[0137] After the above application, a drying process for solvent removal can be performed. The drying process is performed at a temperature and time that can sufficiently remove the solvent, and the conditions may vary depending on the type of solvent and are therefore not particularly limited to the present invention. Examples include drying using warm air, hot air, low-humidity air, vacuum drying, and drying using irradiation with (far) infrared rays and electron beams. The drying speed is usually adjusted to remove the solvent as quickly as possible within a speed range that does not cause cracks in the positive electrode active material layer due to stress concentration or cause the positive electrode active material layer to peel off from the positive electrode current collector.

[0138] Additionally, the density of the positive electrode active material within the positive electrode can be increased by pressing the entire body after drying. Pressing methods include mold pressing and roll pressing.

[0139] The porosity of the positive electrode manufactured by the composition and manufacturing method described above, specifically the positive electrode active material layer, may be 50 to 80%, specifically 60 to 75%. If the porosity of the positive electrode is less than 50%, the filling degree of the positive electrode slurry composition including the positive electrode active material, the conductive agent, and the binder becomes excessively high, so that sufficient electrolyte capable of exhibiting ionic and / or electrical conductivity cannot be maintained between the positive electrode active materials, which may result in a deterioration in the output characteristics or cycle characteristics of the battery, and a serious problem of a decrease in overvoltage and discharge capacity of the battery. On the other hand, if the porosity of the positive electrode exceeds 80% and has excessively high porosity, there is a problem in that the physical and electrical connection with the current collector is lowered, resulting in a decrease in adhesive strength and difficulty in reaction. In addition, the increased porosity may be filled with electrolyte, which may result in a problem in that the energy density of the battery may be lowered, and therefore the porosity is appropriately controlled within the above range.

[0140]

[0141] cathode

[0142] The above negative electrode may include a negative electrode current collector and a negative electrode active material layer applied to one or both surfaces of the negative electrode current collector. Alternatively, the negative electrode may be a lithium metal plate.

[0143] The above negative electrode current collector is for supporting the negative electrode active material layer, as described in the positive electrode current collector.

[0144] The above-mentioned negative electrode active material layer may include a conductive material, a binder, etc. in addition to the negative electrode active material. In this case, the conductive material and the binder follow the above-mentioned.

[0145] The above negative active material is lithium (Li + ) can be reversibly intercalated or deintercalated, a material that can react with lithium ions to form a reversibly lithium-containing compound, lithium metal, a lithium alloy, or a mixture thereof.

[0146] The above lithium ion (Li + ) can be reversibly inserted or de-inserted, for example, crystalline carbon, amorphous carbon, or a mixture thereof. The lithium ion (Li + ) can be, for example, tin oxide, titanium nitrate or silicon. The lithium alloy can be, for example, an alloy of lithium (Li) and a metal selected from the group consisting of sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al) and tin (Sn).

[0147] Preferably, the negative active material may be lithium metal, and specifically, may be in the form of a lithium metal thin film or lithium metal powder.

[0148]

[0149] membrane

[0150] The separator separates or insulates the positive and negative electrodes from each other and enables lithium ion transport between the positive and negative electrodes. It may be made of a porous non-conductive or insulating material, and can be used without any special restrictions as long as it is commonly used as a separator in a lithium secondary battery. The separator may be an independent member such as a film, or may be a coating layer added to the positive and / or negative electrodes.

[0151] It is preferable that the above separator have low resistance to ion movement of the electrolyte and excellent moisture absorption capacity for the electrolyte.

[0152] The above separator may be formed of a porous substrate. Any porous substrate commonly used in secondary batteries may be used as the porous substrate. A porous polymer film may be used alone or in a laminated manner. For example, a nonwoven fabric or a polyolefin porous film made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. may be used, but is not limited thereto.

[0153] The material of the porous substrate is not particularly limited in the present invention, and any porous substrate commonly used in electrochemical devices can be used. For example, the porous substrate may be a polyolefin such as polyethylene, polypropylene, etc., a polyester such as polyethyleneterephthalate, polybutyleneterephthalate, polyamide, polyacetal, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylenesulfide, polyethylenenaphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, It may include at least one material selected from the group consisting of poly(p-phenylene benzobisoxazole) and polyarylate.

[0154] The thickness of the porous substrate is not particularly limited, but may be 1 to 100 μm, preferably 5 to 50 μm. The thickness range of the porous substrate is not limited to the aforementioned range, but if the thickness is excessively thinner than the aforementioned lower limit, the mechanical properties may deteriorate, and the separator may be easily damaged during battery use.

[0155] The average diameter and porosity of the pores present in the porous substrate are not particularly limited, but may be 0.001 to 50 ㎛ and 10 to 95%, respectively.

[0156] The lithium-sulfur battery according to the present invention can be manufactured by laminating (stacking) and folding a separator and electrode in addition to the general winding process.

[0157] The shape of the above lithium sulfur battery is not particularly limited, and can be made into various shapes such as cylindrical, stacked, coin-shaped, and pouch-shaped.

[0158]

[0159] Hereinafter, preferred examples are presented to help understand the present invention, but the following examples are only illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and technical idea of ​​the present invention, and it is also natural that such changes and modifications fall within the scope of the appended patent claims.

[0160]

[0161] <Manufacturing of electrolyte for lithium-sulfur batteries>

[0162] Examples 1 to 5 and Comparative Examples 1 to 8

[0163] An electrolyte for a lithium-sulfur battery was manufactured with the composition shown in Table 1 below.

[0164] Lithium salt 1 Lithium salt 2 Total concentration of lithium salt 1 and lithium salt 2 Non-aqueous solvent Additive type Concentration type Concentration type Concentration Example 1 LiFSI 0.5 M LiTf 0.1 M 0.6 M 2-MeF: DME (1:4 (v / v)) LiNO 3 3 wt% Example 2 0.75 M 0.1 M 0.85 M 5 wt% Example 3 0.75 M 0.05 M 0.80 M 5 wt% Example 4 0.75 M 0.2 M 0.95 M 5 wt% Example 5 0.75 M 0.1 M 0.85 M 3 wt% Comparative Example 1 0.5 M 0.5 M 3 wt% Comparative Example 2 0.75 M 0.75 M 5 wt% Comparative Example 30.75MLiTf0.5M1.25M5wt% Comparative Example 40.20M0.1M0.3M3wt% Comparative Example 51M0.1M1.1M3wt% Comparative Example 6LiTFSI0.5M0.1M0.6M3wt% Comparative Example 7LiBETI0.5M0.1M0.6M3wt% Comparative Example 8--0.5M0.5M3wt%

[0165]

[0166] The above LiFSI is lithium bis(fluorosulfonyl)imide, LiN(SO2F)2

[0167] LiTFSI is Lithium bis(trifluoromethanesulfonyl)imide, LiN(SO2CF3)2

[0168] LiBETI is lithium bis(pentafluoroethanesulfonyl)imide, LiN(SO2CF2CF3)2

[0169] 2-MeF is 2-methylfuran,

[0170] DME is dimethoxyethane.

[0171]

[0172] Capacity retention evaluation of lithium-sulfur batteries

[0173] A positive electrode slurry composition was prepared by mixing 96 wt% of a sulfur-carbon composite (S:C = 75:25 (weight ratio)) as a positive electrode active material and 4 wt% of LiPAA (Lithium Polyacrylate) as a binder. The positive electrode slurry composition was applied to an aluminum current collector and then dried to prepare a positive electrode. The porosity of the prepared positive electrode was about 75%, and the loading was about 3.0 mAh / cm. 2 It was.

[0174] Lithium metal with a thickness of 35 ㎛ was used as the cathode.

[0175] The positive and negative electrodes were positioned so as to face each other, and a polyethylene separator having a thickness of 12 ㎛ and a porosity of about 40% was interposed therebetween. Then, the electrolytes manufactured in Examples 1 to 5 and Comparative Examples 1 to 8 were injected, respectively, to manufacture a lithium-sulfur battery in the form of a pouch cell so that the weight ratio of electrolyte:sulfur was 3:1.

[0176]

[0177] For the lithium-sulfur battery manufactured as described above, the capacity retention rate was compared by performing an initial discharge of 0.1C at 25°C under a cut-off condition of 1.8V - 2.5V, two 0.1C / 0.1C charge / discharge cycles, and then 200 0.2C / 0.5C charge / discharge cycles, and the results are shown in Figures 1 to 5.

[0178]

[0179] From the results of Figure 1, it can be confirmed that Example 1 has a superior capacity retention rate compared to Comparative Example 1, which does not include a triflate compound.

[0180] As shown in the results of FIG. 2, Examples 2 to 4 show superior capacity retention rates compared to Comparative Example 2, which does not include a triflate compound, and Comparative Example 3, which is outside the total concentration range of the lithium salt presented in the present invention. In addition, Comparative Example 3 shows problems of increased viscosity and increased overvoltage of the electrolyte due to the inclusion of a larger amount of the triflate compound than the other examples, and also shows inferior capacity retention rates due to the hindrance of lithium ion movement.

[0181] From the results of FIG. 3, it can be confirmed that Examples 1 and 5 have superior discharge capacity and capacity retention rate compared to Comparative Examples 4 and 5, which are outside the total concentration range of the lithium salt presented in the present invention.

[0182] From the results of FIG. 4, it can be confirmed that Example 1 has superior discharge capacity and capacity retention rate compared to Comparative Examples 6 and 7, which do not include LiFSI as a lithium salt and include other types of lithium salts.

[0183] From the results in Fig. 5, it can be confirmed that Comparative Example 8, which does not include LiFSI as a lithium salt and only includes a triflate compound, has a significantly low discharge capacity. In addition, it can be confirmed that Comparative Example 8 has a low capacity retention rate, as the discharge capacity begins to decrease after 70 cycles.

[0184]

[0185] From the above results, it was found that the electrolyte for a lithium-sulfur battery of the present invention can improve the lifespan of a lithium-sulfur battery by exhibiting excellent capacity retention.

Claims

1. An electrolyte for a lithium sulfur battery comprising a lithium salt, a non-aqueous solvent and an additive. The lithium salts include LiFSI (lithium bis(fluorosulfonyl)imide) and triflate compounds. An electrolyte for a lithium-sulfur battery, wherein the total molar concentration of LiFSI (lithium bis(fluorosulfonyl)imide) and triflate compounds in the electrolyte is 0.5 to 1.0 M.

2. In paragraph 1, An electrolyte for a lithium-sulfur battery, characterized in that the molar concentration of the triflate compound in the electrolyte is 0.05 to 0.30 M.

3. In paragraph 1, An electrolyte for a lithium-sulfur battery, characterized in that the total molar concentration of LiFSI (lithium bis(fluorosulfonyl)imide) and the triflate compound in the electrolyte is 0.6 to 0.95 M.

4. In paragraph 1, An electrolyte for a lithium-sulfur battery, characterized in that the concentration of the triflate compound in the electrolyte is 0.05 to 0.20 M.

5. In paragraph 1, An electrolyte for a lithium-sulfur battery, characterized in that the above triflate compound includes LiTf (lithium triflate).

6. In paragraph 1, An electrolyte for a lithium sulfur battery, characterized in that the additive comprises a nitric acid compound.

7. In paragraph 6, The above nitric acid compound is lithium nitrate (LiNO 3 ) An electrolyte for a lithium sulfur battery.

8. In paragraph 1, An electrolyte for a lithium sulfur battery, characterized in that the non-aqueous solvent comprises an ether solvent.

9. In paragraph 8, An electrolyte for a lithium-sulfur battery, characterized in that the ether solvent is contained in a content of 80% by volume or more based on the total volume of the non-aqueous solvent.

10. In paragraph 8, An electrolyte for a lithium sulfur battery, characterized in that the ether solvent comprises a linear ether, a cyclic ether, or a mixture thereof.

11. A lithium sulfur battery comprising a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, A lithium-sulfur battery, characterized in that the electrolyte is an electrolyte for a lithium-sulfur battery according to any one of claims 1 to 10.

12. In paragraph 11, A lithium sulfur battery, characterized in that the positive electrode includes a sulfur-containing compound as a positive electrode active material.

13. In paragraph 12, The above sulfur-containing compound is inorganic sulfur (S 8 ), lithium polysulfide (Li 2 S n , 1≤n≤8), a sulfur-carbon complex or a mixture of two or more thereof.

14. In paragraph 11, A lithium-sulfur battery, characterized in that the negative electrode comprises lithium metal, lithium alloy or a mixture thereof as a negative electrode active material.

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